A charged particle microscope is used to record an image of an object by scanning a focused charged particle beam across a surface of the object and by detecting secondary particles released from the object by the incident beam in dependence of a scanning position of the beam. In order to obtain a high quality image, it is necessary to correctly focus the charged particle beam on the object surface. The charged particle beam is well focused when the beam forms a small circular beam spot on the object surface. The charged particle microscope typically comprises at least one focusing lens, an x-stigmator and an y-stigmator traversed by the particle beam. The at least one focusing lens includes an objective lens, and the particle beam is focused at a working distance from the objective lens. The focusing of the particle beam on the object can be adjusted by changing the excitations of the at least one focusing lens, the x-stigmator and the y-stigmator and by changing a distance at which the object surface is positioned from the objective lens. The x- and y-stigmators affect an astigmatism of the beam such that the shape of the beam spot formed on the object surface deviates from the circular shape if the x- and y-stigmators are not correctly excited even when the object surface is positioned at the working distance from the objective lens.
In practice, it is difficult to find the optimum excitations of the x- and y-stigmators and of the focusing lens at a given distance of the object surface from the objective lens since the size of the beam spot on the object surface is a complicated non-linear function of the excitations of the at least one focusing lens and the x- and y-stigmators. Therefore, even experienced users require a significant amount of time to obtain a satisfying focusing of the particle beam in an iterative procedure in which the excitations of the at least one focusing lens and the x- and y-stigmators are changed according to a trial and error method.
The present invention has been made taking the above considerations into account.